1. Field of the Invention
The present invention relates to zoom lenses. In particular, the present invention relates to a zoom lens useful as an imaging optical system for video cameras, cameras for silver-halide photographs, digital cameras, and the like.
2. Description of the Related Art
Recently, zoom lenses are required to be compact and provide a high-resolution optical performance, to serve as an imaging optical system for an image pickup apparatus, such as a video camera, and a digital still camera, which uses a solid-state image pickup element.
Also, such a zoom lens is required to have an image stabilizing function for preventing a blur of a captured image caused by a shake applied to the zoom lens when an object image is captured.
To meet these demands, the zoom lens may be a four-unit structure having a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of positive refractive power, the lens units being disposed in that order from an object side to an image side. There is provided a configuration that the second and fourth lens units of such a four-unit zoom lens are moved during zooming, and one of the lens units is shifted for correcting the image blur. In this configuration of the zoom lens, the third lens unit is divided into a lens sub-unit of negative refractive power and a lens sub-unit of positive refractive power. The lens sub-unit of positive refractive power is vibrated in a direction orthogonal to the optical axis thereof, to obtain a still image. For example, such a zoom lens is disclosed in U.S. Pat. No. 6,473,231, and Japanese Patent Laid-Open Nos. 7-128619, 2002-244037, and 2003-322795.
In general, an amount of movement of each lens unit for zooming is decreased as the refractive power of each lens unit in the zoom lens is increased. Accordingly, the entire lens system can be downsized. However, it is difficult to obtain a high optical performance because of increased variation in aberration.
In particular, to obtain a high zoom ratio, the effective diameter of the first lens unit is increased, and thus, the total length of the lenses is increased, this causes the entire lens system to be enlarged.